1. Field of the Invention
This invention relates generally to doctors used in papermaking and web converting machines and is concerned in particular with the provision of a composite doctor blade.
2. Description of the Prior Art
Doctor blades contact the surface of rolls in papermaking and web converting machines for the purpose of cleaning or sheet removal. Conventional doctor blade materials include metals, homogeneous plastics, and cotton, glass and carbon thermoset laminates. Metal blades, although inexpensive, can cause damage or premature wear with some roll surfaces. Many of the water removal rolls on the wet end have a relatively soft rubber cover measured at 10 P&J and softer. Typically, these rolls are doctored with homogeneous plastic doctor blades, such as ultra-high molecular weight polyethylene (“UHMW polyethylene”).
Plastic blades are generally thick, ⅛″ to ¼″, compared to metals and glass and carbon thermoset laminates which are typically 0.050″ to 0.080″ thick. The additional thickness of the homogeneous plastics is required to achieve sufficient machine-direction rigidity for reliable doctoring. However, when compared to thinner blades, a thick doctor blade suffers a loss in doctoring performance due to the decreased contact pressure between the blade and the roll. Cotton laminates, like homogeneous plastics, are relatively thick and have the associated disadvantages. To eliminate some of the problems associated with thick plastic or cotton blades, glass and carbon laminates are often used. Glass laminates are rigid and can therefore be made thin for efficient doctoring. The main disadvantage with glass laminates is that the glass wear particles produced are abrasive, which can unintentionally alter or cause excessive wear with some roll surfaces, especially soft wet end rolls. Carbon laminates have the advantage of being very rigid and non-abrasive. However, carbon laminates are expensive and not always economical for widespread use in the wet end of the paper machine. Therefore, it was necessary to develop a thin and non-abrasive composite doctor blade to efficiently doctor soft wet end rolls on the paper machine.
Coater blades may be in contact with a moving sheet of paper or other material. At the point of blade contact, the sheet is supported by a rotating coater backing roll. The coater blade is used in a wiping mode to meter a layer of liquid on to a sheet of paper or other material. With some coating machines, the coater blade meters a layer of liquid directly on a roll surface and then the coating is transferred to the sheet of material at a later point. The coater blade is held in a holder similar to a doctor blade. The metered liquid or coating thickness is dependent on the amount of pressure applied to the trailing edge of the coater blade. An increased amount of pressure will decrease the coating thickness. Similarly, a decreased amount of pressure will increase the coating thickness. Conventional coater blade materials include steel, stainless steel and steel with a treated edge for prolonging the useful life.
U.S. Pat. No. 4,5499,933 shows a doctor blade having non-homogeneous stiffness properties. U.S. Pat. No. 2,767,529 shows a polishing doctor blade composed of alternate frangible and wear resisting laminations of mildly abrasive and non-abrasive materials, respectively.